.\"
.\" Copyright (c) 2010-2011 The FreeBSD Foundation
.\"
.\" This documentation was written at the Centre for Advanced Internet
.\" Architectures, Swinburne University of Technology, Melbourne, Australia by
.\" David Hayes under sponsorship from the FreeBSD Foundation.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
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.Dd May 13, 2021
.Dt CC_CHD 4
.Os
.Sh NAME
.Nm cc_chd
.Nd CHD Congestion Control Algorithm
.Sh DESCRIPTION
CHD enhances the HD algorithm implemented in
.Xr cc_hd 4 .
It provides tolerance to non-congestion related packet loss and improvements to
coexistence with traditional loss-based TCP flows, especially when the
bottleneck link is lightly multiplexed.
.Pp
Like HD, the algorithm aims to keep network queuing delays below a particular
threshold (queue_threshold) and decides to reduce the congestion window (cwnd)
probabilistically based on its estimate of the network queuing delay.
.Pp
It differs from HD in three key aspects:
.Bl -bullet
.It
The probability of cwnd reduction due to congestion is calculated once per round
trip time instead of each time an acknowledgement is received as done by
.Xr cc_hd 4 .
.It
Packet losses that occur while the queuing delay is less than queue_threshold
do not cause cwnd to be reduced.
.It
CHD uses a shadow window to help regain lost transmission opportunities when
competing with loss-based TCP flows.
.El
.Sh MIB Variables
The algorithm exposes the following tunable variables in the
.Va net.inet.tcp.cc.chd
branch of the
.Xr sysctl 3
MIB:
.Bl -tag -width ".Va queue_threshold"
.It Va queue_threshold
Queueing congestion threshold (qth) in ticks.
Default is 20.
.It Va pmax
Per RTT maximum backoff probability as a percentage.
Default is 50.
.It Va qmin
Minimum queuing delay threshold (qmin) in ticks.
Default is 5.
.It Va loss_fair
If 1, cwnd is adjusted using the shadow window when a congestion
related loss is detected.
Default is 1.
.It Va use_max
If 1, the maximum RTT seen within the measurement period is used as the basic
delay measurement for the algorithm, otherwise a sampled RTT measurement
is used.
Default is 1.
.El
.Sh SEE ALSO
.Xr cc_cdg 4 ,
.Xr cc_cubic 4 ,
.Xr cc_dctcp 4 ,
.Xr cc_hd 4 ,
.Xr cc_htcp 4 ,
.Xr cc_newreno 4 ,
.Xr cc_vegas 4 ,
.Xr h_ertt 4 ,
.Xr mod_cc 4 ,
.Xr tcp 4 ,
.Xr khelp 9 ,
.Xr mod_cc 9
.Rs
.%A "D. A. Hayes"
.%A "G. Armitage"
.%T "Improved coexistence and loss tolerance for delay based TCP congestion control"
.%J "in 35th Annual IEEE Conference on Local Computer Networks"
.%D "October 2010"
.%P "24-31"
.Re
.Sh ACKNOWLEDGEMENTS
Development and testing of this software were made possible in part by grants
from the FreeBSD Foundation and Cisco University Research Program Fund at
Community Foundation Silicon Valley.
.Sh HISTORY
The
.Nm
congestion control module first appeared in
.Fx 9.0 .
.Pp
The module was first released in 2010 by David Hayes whilst working on the
NewTCP research project at Swinburne University of Technology's Centre for
Advanced Internet Architectures, Melbourne, Australia.
More details are available at:
.Pp
http://caia.swin.edu.au/urp/newtcp/
.Sh AUTHORS
.An -nosplit
The
.Nm
congestion control module and this manual page were written by
.An David Hayes Aq Mt david.hayes@ieee.org .
